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a
c
b
−q
m
−q
m
−q
m
−2q
m
−2q
m
−2q
m
+q
m
+q
m
+q
m
+2q
m
+2q
m
+2q
m
d e
carry a quantized amount of energy, momen-
tum, electric charge and spin. In their theoreti-
cal study, Castelnovo et al. find the first instance
of such an excitation with a non-zero magnetic
charge. Under certain conditions, these mag-
nets behave as a gas of independent magnetic
poles. There is even a phase transition at which
a thin vapour of these monopoles condenses
into a dense liquid.
How a monopole can be created in a world
of magnetic dipoles can be understood by
considering a one-dimensional string made
by laying tiny dipoles end to end. In this case, a
single misaligned dipole gives rise to two inde-
pendent magnetic charges that can be moved
far apart, for the price of putting some energy
into the system (Fig. 1a–c). The monopoles
that arise are boundaries separating regions
with perfectly aligned dipoles. These topo-
logical defects, known as domain walls, or
‘kinks’, have recently been studied in magnetic
nanowires
3
.
The emergence of free magnetic monopoles
is an example of the phenomenon known as
‘fractionalization’: that the collective behaviour
of many particles in a condensed-matter sys-
tem is most effectively described in terms of
fractions of the original particles. Fractionali-
zation is often tied to topological defects
4
and
is common in one-dimensional systems, such
as the string already mentioned. The only con-
firmed case in two dimensions is the fractional
quantum Hall effect, which occurs in a cold gas
of electrons placed in a strong magnetic field
5
.
Measurements of conductance
6
and electrical
noise
7
in this system indicate the involvement
of ‘quasi particles’ with one-third of an elec-
tron’s charge.
Castelnovo and colleagues provide the
first example of fractionalization in a three-
dimensional system. But how does the phys-
ics of free monopoles on a string survive in a
higher-dimensional setting? The answer lies in
the special nature of the ground states of the
authors’ chosen system, spin ice, which allows
one-dimensional ideas to be transferred to two
and three dimensions (Fig. 1d,e).
The monopoles in spin ice are magnetic
analogues of electrically charged defects H
3
O
+
and OH
−
in water ice. The movement of these
defects through water ice causes it to conduct
electricity when an electric field (potential dif-
ference) is applied across it. Might it be possible
to create a steady magnetic current in spin ice
by placing it in a magnetic field? Unfortunately
not. The motion of a kink alters the state of a
string, making it impassable to the next mag-
netic charge. In water ice, a kink of a different
flavour, known as a Bjerrum defect
8
, repairs the
damage done by the original defect. Because
there is no analogue of Bjerrum defects in spin
ice, magnetic monopoles are somewhat limited
in their motion, and cannot sustain a direct
magnetic current.
That still leaves the possibility of generating
an alternating magnetic current, which would
be interesting in its own right. In any case,
learning how to move magnetic monopoles
around would be a step towards technologies
such as magnetic analogues of electric circuits
and magnetic memories
9
operating on the
atomic scale. ■
Oleg Tchernyshyov is in the Department
of Physics and Astronomy, Johns Hopkins
University, 3400 North Charles Street, Baltimore,
Maryland 21218, USA.
e-mail: olegt@jhu.edu
1. Castelnovo, C., Moessner, R. & Sondhi, S. L. Nature 451,
42–45 (2008).
2. Bramwell, S. T. & Gingras, M. J. P. Science 294, 1495–1501
(2001).
3. Saitoh, E., Miyajima, H., Yamaoka, T. & Tatara, G. Nature
432, 203–206 (2004).
4. Goldstone, J. & Wilczek, F. Phys. Rev. Lett. 47, 986–989
(1981).
5. Laughlin, R. B. Phys. Rev. Lett. 50, 1395–1398 (1983).
6. Tsui, D. C., Stormer, H. L. & Gossard, A. C. Phys. Rev. Lett.
48, 1559–1562 (1982).
7. de-Picciotto, R. et al. Nature 389, 162–164 (1997).
8. Petrenko, V. F. & Whitworth, R. W. in Physics of Ice 73–76
(Oxford Univ. Press, 1999).
9. Thomas, L. et al. Nature 443, 197–200 (2006).
10. Wang, R. F. et al. Nature 439, 303–306 (2006).
Figure 1 | Making monopoles. a, In the lowest-energy state, all the elements
in a chain of magnetic dipoles point in the same direction: the north pole
(magnetic charge +q
m
) of one magnet touches the south pole (magnetic
charge −q
m
) of the next. The charges cancel out all the way along the
string, except at the ends. b, Flipping one of the dipoles in the middle
excites the chain out of its ground state, creating two magnetic charges
+2q
m
and −2q
m
. c, Each of these charges can be moved independently of
the other by flipping a dipole next to it — they are free monopoles.
d, Castelnovo et al.
1
study spin orientations in spin ice. Shown here is square
spin ice, a two-dimensional variant that has been produced in an artificial
form, as an array of nanoscale magnets
10
. Four magnetic poles meet at each
point on the square lattice, and the energy is lowest when two are north
poles and two are south poles. Spin ice in the ground state can be construed
as a series of strings of magnetic dipoles embedded in a higher-dimensional
lattice. e, Flipping dipoles on a single string (the black one) creates a pair of
well-separated magnetic poles just as in one dimension.
AQUACULTURE
The price of lice
Andrew A. Rosenberg
Wild salmon stocks in Canadian coastal waters are being severely affected
by parasites from fish farms. So intense are these infestations that some
populations of salmon are at risk of extinction.
The global demand for fish is on the rise, and
farmed sources are taking much of the strain
— the catch of wild fish has levelled off, and
may well be declining
1
, but aquaculture pro-
duction is expanding rapidly
2
. The ecological
costs of that expansion can be heavy, however,
as Krkošek et al. show in Science
3
. The message
of their paper is that there are some serious
issues that cannot be ignored if the expansion
of aquaculture is to be productive rather than
destructive.
Consumers can readily see the shift towards
aquaculture, particularly for products such as
farmed salmon, which has become a staple of
supermarkets and restaurants in Europe and
North America. Those buying fish will be aware
of press reports of overfishing and resource
depletion. Some may even look for eco-labels
or carry a little card to guide them towards the
purchase of sustainable products. As my col-
league Carl Safina has said, “Give a man a fish
and you have fed him for a day. Give a man a
seafood choice card and you have made him
impossible to dine with.”
But aquaculture products tend to be sub-
ject to less public attention, even as issues
23
NATURE|Vol 451|3 January 2008
NEWS & VIEWS
ranging from habitat destruction to
the effects of using wild fish to feed
farmed stocks
4
become of greater
concern. The emphasis of aquacul-
ture development has, not surpris-
ingly, been on increasing production,
lowering costs and improving prod-
ucts. These needs of the industry
have been well served by the sci-
ence of fish farming. Unfortunately,
however, research pointing out the
environmental costs of production
has been viewed as an attack on the
industry, rather than as a challenge to
be tackled and overcome.
Krkošek et al.
3
have provided new,
empirical evidence of the environ-
mental costs of the nearshore, net-
pen aquaculture of salmon — the
pending extinction of several populations of
wild pink salmon, Oncorhynchus gorbuscha,
on the coast of British Columbia. Until now,
most research on the effects of net-pen aquac-
ulture has revealed instances of certain conse-
quences, such as the competition of wild fish
with escaped farm stocks for spawning habi-
tat, but not of effects at the population level.
Krkošek and colleagues’ analysis of 142 popu-
lations of pink salmon shows that wild stocks
adjacent to fish farms have suffered dramatic
increases in mortality of juvenile fish owing to
infestation by sea lice, Lepeophtheirus salmonis
(Fig. 1), and that most of the exposed popu-
lations are at risk of extinction within four
salmon generations. It is the location of the
salmon farms, compounded with the tendency
of sea lice to proliferate near intensive farm
facilities, that are cause for concern. Although
farmed salmon can be treated to reduce sea-
lice infestation, wild stocks have no such pro-
tection. As juvenile wild salmon emerge from
rivers, their migration route runs a gauntlet of
salmon aquaculture pens and, therefore, high
levels of salmon lice.
The lesson of this analysis
3
is that neither
fisheries science nor the aquaculture industry
can be driven solely by the desire to increase
production. Many aquaculture facilities are
set in complex ecosystems, and influence the
structure and function of those ecosystems.
Policy-makers must ensure that the environ-
mental costs are evaluated and monitored, and
are factored into decisions about farm location
and expansion. The argument that costs will be
unfairly passed on to consumers rings hollow,
because if such costs are not controlled, we the
public eventually bear them in their entirety. In
such a circumstance, there is little incentive for
producers to reduce environmental impacts.
The results of Krkošek and colleagues’ study,
and their warning that the terrible cost of
extinction of wild-salmon stocks is not far off,
highlights that point.
Net-pen aquaculture is not just about
salmon. As the industry expands into other
species, such as cod, halibut and sablefish,
the same concerns over the location of farms,
NEUROSCIENCE
Love hangover
Leslie C. Griffith
In many species, males have developed strategies to safeguard their
genetic material from dilution by that of competing males. Fruitflies achieve
this by altering the behaviour of their partners.
Sex can be transformative. Humans often
romanticize the after-effects of copulation, but
for most organisms there are real biological
consequences to mating that go beyond the
transfer of sperm. Most species have strategies
for protecting their genetic investment that
can involve alterations in both the biology and
behaviour of the mating partners. For example,
in the fruitfly Drosophila melanogaster a com-
ponent of seminal fluid, known as sex peptide,
leads to increased egg-laying by the mated
female and behavioural changes that reduce the
likelihood of her re-mating. How sex peptide
triggers such a complex array of effects was
unknown. On page 33 of this issue, Yapici
et al.
1
identify the receptor for sex peptide and
show that it is expressed in the reproductive
tract and in a subset of female neurons believed
to be involved in sexual behaviour.
Enhancing the survival of potential progeny
is a common goal of males in many species. In
mammals, intercourse changes the immuno-
logical environment of the female reproductive
tract, increasing the probability of successful
fertilization and implantation
2
. This type of
post-copulatory effect benefits both the male
and female partner. For many species there are
also other mating-associated events that appar-
ently maximize the reproductive success of just
one of the involved parties, often at the expense
of the other. An obvious example of this is mate
guarding. Males of many avian, reptile, rodent,
primate and insect species remain close to a
recent conquest to lower the probability of her
re-mating with a more desirable male and so
diluting or displacing their own sperm. The
female may benefit in terms of decreased pre-
dation, but she loses any opportunity to better
the genetic lot of her offspring. The success
of this strategy for the male depends on
his vigilance, and potentially decreases his
chances of mating with other females, so is not
without cost.
Nature has also come up with more subtle
forms of mate guarding. In snakes
3
and various
insect species
4–6
, mating can lead to changes
disease and parasite transmission, and other
impacts will certainly apply to these species too.
It is vital to assess the potential environmental
costs and to reduce them before the advent of
large-scale farming of these species.
Can we design aquaculture systems that
reduce ecosystem impacts, or eliminate some
of them entirely? I think that we can, but not
with a confrontational mentality. Overfishing
of wild stocks is not a contrived problem, nor
is it unsolvable: good management practices
BLICKWINKEL/HARTL
are often contentious and difficult
to implement, but ultimately they
can work. Similarly, the problems
facing the aquaculture industry
are not unsolvable, but denial that
those problems exist will not pro-
vide answers. In the case of salmon
lice, solutions include adhering to
strict guidelines on the introduc-
tion and transfer of non-native
fish, and siting of net pens away
from areas where wild stock is
vulnerable. Overall, the priority in
aquaculture should be to anticipate
any adverse environmental con-
sequences and to tackle them at
that stage, rather than struggle to
recover after those consequences
are already apparent. ■
Andrew A. Rosenberg is at the Institute for the
Study of Earth, Oceans and Space, University
of New Hampshire, Durham, New Hampshire
03824-3525, USA.
e-mail: a
ndy.rosenberg@unh.edu
1. Watson, R. & Pauly, D. Nature 414, 534–536 (2001).
2. Food & Agriculture Organization of the United Nations
State of World Aquaculture 2006 Fisheries Tech. Pap. 500
(FAO, Rome, 2006).
3. Krkošek, M. et al. Science 318, 1772–1775 (2007).
4. Naylor, R. L. et al. Issues Ecol. No. 8 (2001).
Figure 1 | Salmon louse. This marine organism belongs to a group of
crustaceans known as copepods. It feeds on the external surfaces of fish,
and can eventually kill them.
24
NATURE|Vol 451|3 January 2008
NEWS & VIEWS
